The anterior chamber of the eye is filled with a fluid known as aqueous humor. This fluid is produced by the ciliary body located adjacent to the lens. The fluid flows around the lens, through the trabecular meshwork and into an anatomical conduit, the sinus venosus sclerae, also known as Schlemm's Canal. The fluid is then expelled from the canal to the episcleral veins. In the disease state known as narrow or open angle glaucoma, the flow through the trabecular meshwork is reduced, thereby causing increased intraocular pressure, which can lead to degradation of the optic nerve and blindness. The outflow of the trabecular meshwork is typically blocked by structural or cellular debris, which is primarily age related. Various methods have been tried and used to lower the intraoptic pressure in the management of glaucoma.
Standard medical treatment for glaucoma involves topically applied drugs, including beta blockers, such as Timoptic, alpha adrenergic agonists and prostaglandin analogues. The goal of these drugs is to dilate the flow tracts in the trabecular meshwork thereby increasing flow, or to reduce the production of aqueous humor by the ciliary body. The success rate of drug treatment can range widely due to variabilities in patient response and drug side effects. These drugs are primarily delivered via drops applied directly to the eye one or two times per day. These medical treatments must also take into account patient compliance and the high cost of the drugs over time.
Surgical treatment for glaucoma has been evolving over the past two decades and includes trabeculostomy, laser trabeculoplasty, implantable shunts and viscocanalostomy. Trabeculostomy and laser trebeculoplasty involves opening or creating larger channels through the trabecular meshwork, either by surgical dissection or laser ablation of the tissues. Although effective for a short period, long term follow-up of these treatments shows marked increases in intraocular pressure and therefore low success rates. Implantable shunts, which carry the aqueous humor from the anterior chamber directly to the sclera, require precise surgical implantation and are primarily used as a last resort when all other treatment fails. Typical such devices are disclosed by Brown, et al. in U.S. Pat. No. 5,743,868 and by Wandel in U.S. Pat. No. 5,807,302. The use of shunts also involves increased surgical complications, such as infection, blebs (fluid pockets forming in the sclera at the outflow of the shunt) and blockage of the shunt over time.
A recently developed surgical treatment is known as viscocanalostomy. The procedure involves surgically opening a flap of the sclera and dissecting down to de-roof Schlemm's Canal to increase aqueous humor drainage. A high viscosity viscoelastic material is injected into the canal to dilate it, and may act to open the trabecular meshwork from the canalicular space. The viscoelastic material also acts as a fibrosis inhibitor, reducing the influx of fibroblastic cells from the healing response, which would negate the effects of the procedure by blocking fluid flow. Stegmann, et al. in U.S. Pat. No. 5,486,165 disclose a microcannula designed for delivery of substances to Schlemm's Canal during this procedure. In EP 089847A2, Grieshaber, et al. disclose an improvement to the Stegmann apparatus to deliver substances or stents for maintaining the passage of fluid in the canal. Both of these prior art citations are directed at the surgical case. While the procedure appears promising for the treatment of glaucoma, considerable training and skill is required of the practitioner. The procedure requires the precise dissection of the scleral layers and delicate manipulation of the canal, and therefore carries the concomitant risks of ocular surgery, such as infection, hypotony and endophthalmitis.
Imaging of the structure of the eye has been accomplished by various means. Ultrasound imaging is routinely used in the management of ocular disease, and such ultrasound scanners are available commercially. Ultrasound scanners normally operate in the frequency range of 10-20 MHz. The resolution at these frequencies is insufficient to provide tissue discrimination of fine structures associated with the anterior chamber such as Schlemm's Canal. Recently, higher frequency imaging systems have been developed as ultrasound biomicroscopes, e.g. P40 UBM, Paradigm Medical Industries, Salt Lake City, Utah. These systems provide imaging in the range of 40-60 MHz and are able to image the fine structure of tissues. High resolution ultrasound imaging of tissues with high frequency is also used in intravascular ultrasound (IVUS) catheters, used for the detection and characterization of vascular disease. Thomas, et al. in U.S. Pat. No. 4,911,170 disclose such a high frequency ultrasonic imaging catheter.
The combination of imaging and percutaneously introduced surgical tools has the potential to convert invasive surgical procedures into rapid minimally invasive methods. For example, image guided biopsy of breast lesions is routinely performed as an alternative to surgical dissection with advantages in reduced surgical time, patient trauma, and infection risk. Guidance under X-ray or ultrasound is used to locate a lesion and then advance a biopsy needle to the site. Many different approaches have been made to attach a biopsy needle to an ultrasound transducer to guide the biopsy needed to the general area that is being imaged. Miller, et al. in U.S. Pat. No. 5,758,650 and Park et al. in U.S. Pat. No. 5,924,992 disclose typical such devices. The prior art is directed at needle guides which are attached to any transducer and are capable of guiding biopsy needles to the target site. These needle guides are disposed at a narrow angle with respect to the transducer axis and therefore are not able to target sites that are directly under the transducer face with adequate accuracy as is required in the case of ophthalmic surgery.
The following references and all other references referred to herein are hereby incorporated by reference in their entirety:
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